[Bug 1889509] Re: grub boot error : "symbol 'grub_calloc' not found

[Ian Channing]

We had a problem with our recovery VM switching the disk names so here's
a minor modification from [comment
#16](https://bugs.launchpad.net/ubuntu/+source/grub2/+bug/1889509/comments/16)
to first run `lsblk` to check for the correct disk before running `mount
/dev/sdc1`:

$ sudo su -
# lsblk <-- this will identify the attached disk, usually /dev/sdc - there should be no mount points on it
# mkdir /rescue
# mount /dev/sdc1 /rescue
# for fs in {proc,sys,tmp,dev}; do mount -o bind /$fs /rescue/$fs; done
# cd /rescue
# chroot /rescue
# grub-install /dev/sdc
# exit
# cd /
# for fs in {proc,sys,tmp,dev}; do umount /rescue/$fs; done
# umount /rescue
# rmdir /rescue

Also this was the refined set of steps that we used (always using the
same recovery VM:

1. Make a snapshot of 'broken' OS disk (postfix `_snap`)
2. Create a Managed Disk from the snapshot - this must be the same grade as the old OS disk (30 GB Premium SSD) as we are going to fully replace the old OS disk with this one (postfix `_recovery`) - source type snapshot and use the just created snapshot
3. Attach Managed OS Disk to recovery VM (stop/start of recoveryVM not required)
4. Login via SSH, run recovery steps, logout again
5. Detach Managed OS Disk from recovery VM (edit the VM disks and detach the recovery OS Disk)
6. Stop the 'broken' VM (possibly not necessary as the OS Disk swap stops it)
7. In the 'broken' VM Disks click 'Swap OS Disk' and select the recovery OS Disk as the replacement
8. Start the 'recovered' VM
9. Clean up the snapshot - but leave the broken OS disk for now - reminder for a month or so to remove it too

Finally turn off the recovery VM and delete that in a month too

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https://bugs.launchpad.net/bugs/1889509

Title:
  grub boot error : "symbol 'grub_calloc' not found

Status in grub2 package in Ubuntu:
  Confirmed
Status in grub2 package in Debian:
  Unknown

Bug description:
  After updating grub2 (to 2.02~beta2-36ubuntu3.26) and rebooting, my
  server does not boot:

  Booting from Hard Disk 0...
  error: symbol `grub_calloc' not found.
  Entering rescue mode...
  grub rescue> _

  I rebooted 3 servers in this way (all running Ubuntu 16.04.6 LTS) and
  all hung.

  A lot of other people are reporting the same problem at:

  https://askubuntu.com/questions/1263125/how-to-fix-a-grub-boot-error-
  symbol-grub-calloc-not-found.

  ---

  Above most likely means that dpkg debconf no longer knows about the
  correct drives to install grub onto.

  Please boot & mount all the target disks and execute

  $ sudo dpkg-reconfigure grub-pc

  You will be asked which drives to install grub onto, and then grub
  will be installed onto them, and more importantly, it will remember
  where to install grub to, on all future upgrades.

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